The word 'incest' was introduced into Middle English around 1225 as a legal term to describe the crime of familial incest as it is known today. It was also used to describe sexual relations between married persons, one of whom had taken a vow of celibacy (often called spiritual incest).[37] It derives from the Latin incestus or incestum, the substantive use of the adjective incestus meaning 'unchaste, impure', which itself is derived from the Latin castus meaning 'chaste'. The derived adjective incestuous does not appear until the 16th century.[38]

Prior to the introduction of the Latin term, incest was known in Old English as sibbleger (from sibb 'kinship' + leger 'to lie') or mǣġhǣmed (from mǣġ 'kin, parent' + hǣmed 'sexual intercourse') but in time, both words fell out of use._________________lorem ipsum